Self-aligned gate field emitter device and methods for producing the same

ABSTRACT

A field emitter and its fabrication method is described in which a gate electrode is formed around and substantially encloses the emitter. The emitter is formed on a silicon substrate and is in the form of a pyramid structure. The surface of the pyramid includes an oxide layer on it. The whole device is baked until the photoresist is drawn, by surface tension, towards the base of the pyramid to expose the metal layer. Etching of the metal layer and the oxide layer produces the finished device which may suitably be employed as a switch in an electronic circuit.

The present invention relates to self-aligned gate field emitter devicesand to methods of producing the same and has particular, although notexclusive, relevance to such devices as may be employed as switches inelectronic circuits.

The concepts of field emission, i.e. the presence of a very thin barrierpotential at a surface from which electrons may migrate, are well known.Numerous devices exist which exhibit field emission. One such example isa sharply pointed substrate such as disclosed in "Atomically SharpSilicon and Metal Field Emitters", IEEE Transactions on ElectronDevices, Vol. 38, No. 10, Oct. 1991. This literature describes a methodfor producing an atomically sharp silicon tip of less than 10°-15°half-angle. It is known that such sharp tips provide the very thinbarrier potential necessary for field emission.

However during the fabrication of a device such as described above,great care needs to be taken to ensure that no damage occurs to thefield emitter. This problem will be appreciated because such structuresare generally microengineered. This term will be understood by thoseskilled in the art as meaning that fabrication is conducted on scales ofaround 1×10⁻⁶ m.

Furthermore if such a device is fabricated with a gate structure, aswill generally be the case when the device is to be employed inelectronic circuitry, then accurately positioning the gate with respectto the field emitter is an arduous task when the device ismicroengineered.

It is thus an object of the present invention to at least alleviate theaforementioned problem.

According to a first aspect of the present invention there is provided aself-aligned gate field emitter device comprising: a substrate carryinga tapered protrusion; the tapered protrusion carrying on electricallyinsulative layer at least partially covering the protrusion, theelectrically insulative material extending along the flanks of thetapered protrusion from the base adjacent the substrate towards the tipof the protrusion remote from the substrate; electrically conductivematerial formed on the electrically insulative layer and extendingfurther towards the tip of the protrusion than the insulative layer andspaced from the protrusion, the tapered protrusion forming the emitterof the device and the electrically conductive material forming the gateof the device, which gate, in operation of the device, provides controlfor the level of field emission from the emitter, characterized in thatelectrically conductive material is partially covered by thermoplasticmaterial substantially around the base of the protrusion for supportingthe electrically conductive material.

Because the electrically conductive material overlies the electricallyinsulative material by some way, then a more rigid device is formed byprovision of the thermoplastic material around the electricallyconductive material.

Advantageously the electrically insulative material is formed byoxidation of the tapered protrusion. This then obviates the need for aseparate coating of insulative material. Alternatively it is possiblefor the insulative material to be an oxide coating formed on theprotrusion. Additionally, the protrusion may be formed from thesubstrate material itself.

According to a further aspect of the present invention there is provideda method of producing a self-aligned gate field emitter devicecomprising: providing a substrate of material from which the fieldemitter is to be produced and forming a tapered protrusion thereon;forming, on the surface of the protrusion, electrically insulativematerial; coating the electrically insulative material with electricallyconductive material; at least partially coating the electricallyconductive material with thermoplastic material; planarizing the devicesuch that the thermoplastic material remains around the base of theprotrusion substantially remote from the tip thereof to at leastpartially expose the electrically conductive material; selectivelyremoving at least part of the electrically conductive material and theelectrically insulative material thereby to define a portion of thedevice substantially surrounding and enclosing the protrusioncharacterized in that the planarizing comprises heating thethermoplastic material so that it flows and settles around the base ofthe protrusion.

The gate is thus actually formed around the emitter and uses the emittershape as a basis for its formation. Furthermore, because of thegeometries employed, it will be apparent that such a technique requiresno separate masking to be employed. It will also be apparent that thismethod allows exposure of the emitter to be prevented until the finalsteps of the fabrication thus reducing the tendency for it to bedamaged.

According to another aspect of the present invention there is provided amethod of producing a self-aligned gate field emitter device inaccordance with the first aspect of the invention comprising: providinga substrate of material from which the field emitter is to be producedand forming a tapered protrusion thereon; forming, on the tip of theprotrusion a cap of the electrically insulative material and furtherforming on the surface of the protrusion, electrically insulativematerial; rotating the device about an axis through the tip of theprotrusion and substantially perpendicular to the base thereof; coatingthe electrically insulative material, off-axis, whilst the device isrotating with electrically conductive material; selectively removing atleast part of the electrically conductive material and the electricallyinsulative material, including the cap, thereby to define a portion ofthe device substantially surrounding and enclosing the protrusion. Thusby coating the protrusion with the electrically conductive materialusing off-axis rotation coating, it is possible to form the conductivematerial substantially along the flanks of the protrusion without theneed to employ a separate mask.

Preferably the formation of electrically insulative material on theprotrusion is achieved by oxidation of the surface of the protrusion.This then obviates the need for a separate coating of insulativematerial. Alternatively it is possible for the insulative material to bean oxide coating formed on the protrusion. Additionally the protrusionmay be formed from the substrate itself.

In accordance with any of the aspects of the present invention, theprotrusion may be formed from a semiconductor and this may be at leastpartially n-type doped. Alternatively, the semiconductor may be n-typedoped at the tip and base regions of the protrusion, and p-type dopedtherebetween.

The invention will now be described, by way of example only withreference to the following drawings, of which:-

FIG. 1-7 illustrate schematically the fabrication stages of, and adevice in accordance with, a first embodiment of the present invention;

FIGS. 8-11 illustrate schematically the fabrication stages of, and adevice in accordance with, a second embodiment of the present invention;

FIGS. 12-15 illustrate schematically the fabrication stages of, and adevice in accordance with, a third embodiment of the present invention;and

FIGS. 16 and 17 illustrate schematically the doping of a substrate toachieve an electronic switch utilising a device in accordance with thepresent invention.

By reference firstly to FIG. 1, the basic structure from which a devicein accordance with the present invention is fabricated will be seen.

The structure consists of a substrate material 2 which may be chosen tobe a semiconductor such as silicon, and supported by the siliconsubstrate 2 is a tapered protrusion such as the pyramid 4. The pyramid 4ultimately forms the emitter of the device as will become apparenthereafter. The pyramid 4 may be formed on the silicon substrate 2 by anyof several ways, each of which will be readily apparent to those skilledin the art, yet such are not germane to the present invention. Forexample, the pyramid may be a polished single crystal silicon disc cuton the 100 axis and either formed on, or formed from the siliconsubstrate. The size of the pyramid 4 from its base to its tip is of theorder 8×10⁻⁶ m, although pyramids 4 of any size may be employed.

FIG. 2 illustrates the next stage in the fabrication of the device. Thepyramid 4 has formed thereon an electrically insulative material such asan oxide layer 6. The oxide layer 6 may be formed either by oxidation ofthe surface of the pyramid 4 or by coating the pyramid 4 with an oxide.Either of these techniques is equally efficacious and are both wellknown to those skilled in the art. However, if the oxide layer 6 isformed by oxidation of the pyramid 4, then, as will be seen from FIG. 2,the tip of the pyramid 4 of silicon, per se, will become sharpened bythis oxidation. This is advantageous as a separate sharpening of thepyramid 4 tip is then obviated.

Reference now also to FIG. 3 shows that the oxide layer 6 is coated withelectrically conductive material such as metal layer 8. This coating maybe applied by any suitable technique, such as sputtering or evaporation.

FIG. 4 illustrates that the metal layer 8 is coated with plasticsmaterial such a polymer of photoresist 10. In the example of FIG. 4,this photoresist 10 coating covers the metal layer 8 entirely and thephotoresist 10 is deposited by any suitable technique, such as spinning.

The next stage of the fabrication of the device as shown in FIG. 5 is tobake the entire device until the photoresist 10 is drawn down thepyramid 4 towards its base by surface tension sufficiently to expose themetal layer 8. The degree to which the metal layer 10 needs to beexposed will depend, as will become apparent, upon the spacingultimately required between the emitter and the gate of the device. Inthe present example, a temperature of around 140° C. is sufficient tomelt a typical positive photoresist material so that the desired effectis achieved.

FIG. 6 illustrates the next stage of fabrication of the device in whichboth the metal layer 8 and the oxide layer 6 are selectively removed toan extent by, for example, etching away. Such removal techniques will bereadily apparent to those skilled in the art and hence will not bereferred to herein.

It will be seen that the oxide layer 6 is etched away further towardsthe base of the pyramid 4 than the metal layer 8. This is because, inthe finished device, the metal layer 8 will form the gate and needs tobe as close as possible to the emitter (formed by the tip of pyramid 4)in order to function effectively. Removal of at least part of the metallayer 8 and the oxide layer 6 also exposes the tip of the pyramid 4.This tip acts as the emitter of the finished device. It will be apparentthat the above fabrication stages leave the emitter of the devicecovered by another material until the final stage of fabrication, thusoffering some protection against accidental damage. Furthermore, it willbe apparent that the gate region (formed by the metal layer 8) has beenautomatically formed in self-alignment with the emitter region by virtueof the above fabrication.

Reference to FIG. 7 illustrates the device described above in use. Ashas been detailed herebefore, the gate region is formed by the metallayer 8 and the emitter by the pyramid 4. A power supply 12 is arrangedto be connected to the emitter and the gate such that the emitter is ata negative potential with respect to the gate, with suitable biassing,electrons will be emitted from the tip of the pyramid 4. The gate, inthis example, acts as a control mechanism determining the level ofemission current. This is altered by simple adjusting of emissioncurrent. This is altered by simply adjusting the difference in potentialbetween the gate and the emitter.

A second embodiment of the present invention will now be described withreference to FIGS. 8-11 in which parts corresponding to those shown inFIGS. 1-7 are correspondingly numbered. The formation of the device upto and including the deposition of the metal layer 8 is as describedbefore. However, the photoresist 10 is deposited in less abundance thanpreviously such that the pyramid 4 stands proud of the photoresist 10and has a portion of the metal layer 8 exposed. On baking the device,the photoresist, by virtue of surface tension, moves away from the tipof the pyramid 4 to cover only the base region thereof, as is shown inFIG. 8.

The metal layer 8 is once again selectively removed by, for example,etching and the photoresist 10 is completely removed by washing in asuitable solvent leaving the pyramid 4 bearing the metal layer 8 only atthe base as is shown in FIG. 9. Next, a metal plating 14 is formed onthe metal layer 8 and at least a part of the oxide layer 6. There arevarious methods known to those skilled in the art which may achievethis, one such method being utilising the metal layer 8 as thedeposition electrode in a metal electroplating bath. Reference to FIG.10 illustrates the effect of forming the metal plating 14. Finally, asbefore, the oxide layer 6 is selectively removed by, for example,etching to leave the finished device of FIG. 11. As with the device ofFIG. 7, the metal plating 14 of FIG. 11 helps to provide support for themetal layer 8 gate structure in regions where it does not overlie theoxide layer 6 and is separate from the emitter tip, and because theplating 14 is an electrical conductor, will also act as the gate intandem with metal layer 8.

Referring now to FIGS. 12-15, a further embodiment of the presentinvention will be described. Referring firstly to FIG. 12, the pyramid 4has formed on its tip, an oxide cap 16. The way in which the cap 16 isformed on the pyramid is not of significance to the present inventionand so will not be described herein. Those skilled in the art will beaware of suitable microengineering techniques apt to achieve thisstructure. Next, as illustrated in FIG. 13, an oxide layer 6 is formedon the surface of the pyramid 4 in the same manner as described above.If the oxide layer 6 is chosen to be formed by oxidation of the silicon,then it will be apparent this process will not effect the cap 16 inanyway, because cap 16 is already an oxide.

FIG. 14 illustrates the next stage of fabrication in which the wholedevice is rotated about an axis formed through the pyramid 4 from itstip to a point substantially perpendicular to its base. Thus it will beseen that rotation about this axis results in a rotation of the pyramid4 about its point of symmetry. As before, a metal layer 8 is coated ontothe oxide layer 6. However, it must be noted that the coating must beperformed off-axis, as illustrated clearly in the Figure. This isnecessary to achieve coating of the oxide layer substantially along theflanks of the pyramid 4. If an on-axis coating were performed, thenthere would be no metal layer 8 deposited on the flanks of the pyramid4.

It will be apparent that the source of the coating to provide the metallayer 8 should be of sufficient distance away from the device to providea substantially collimated beam of coating material. During the coating,the cap 16 acts as a screen to prevent a metal layer 8 being formedaround the tip region of the pyramid 4.

Referring now to FIG. 15, it will be seen that the final stage offabricating the device is to selectively remove by, for example,etching, the oxide layer 6 and cap 16; this stage being essentially thesame as the similar stages described with reference to FIGS. 6 and 11.

The device and methods for fabrication of the device described abovemay, as has been detailed, be employed as a switch in an electroniccircuit. For this employment, it may be advantageous to dope thesubstrate material in order to achieve a more efficient switch.Reference to FIGS. 16 and 17 illustrate this.

Referring firstly to FIG. 17, if the final device as, for example,illustrated in FIG. 6 is arranged to have the silicon doped to ben-type, either before, during or after the fabrication, then when thepower supply 12 is connected to the gate and substrate regionsappropriately, the device may act as a field effect device such as aMOSFET.

Because the gate 8 (formed by the metal layer 8) is, in this example,biassed negatively with respect to the n-type silicon, then a depletionregion 18 is set up adjacent the flank surfaces of pyramid 4. Theelectrons emitted via the tip of pyramid 4 are thus "pinched" throughthe channel defined by the depletion region, as is standard. The gate 8thus controls the electron channel. This is, depending on the relativebiassing of the gate 8 in relation to the n-type silicon, the width ofthe electron channel surrounded by the depletion region 18 may becontrolled, and hence the rate of efflux of electrons from the tip ofthe pyramid 4. With the example illustrated in FIG. 16, it will beapparent that an electrode structure 20, positively biassed, isnecessary in order to attract the electrons emitted from the tip ofpyramid 4, because the gate 8 is negatively biassed.

Referring now to FIG. 16, it will be seen that the tip and base (and theremainder of the silicon substrate) have been doped to be n-type, whilstthe region of the pyramid 4 therebetween has been doped p-type. The gate8 is biassed by power supply 12 to be positive with respect to thesilicon. As will be understood with this MOSFET arrangement, thepositive gate biassing causes an n-type channel 22 to be formed alongthe surface of the flanks of pyramid 4. It is along this channel 22 thatthe electrons are attracted by the attraction of gate 8 and emitted fromthe tip of the pyramid. Those skilled in the art will appreciate thatthe structure of FIG. 17 does not require a further separate electrodestructure to induce field emission.

By employing a device in accordance with either of FIGS. 16 or 17, anefficient switch is formed as compared generally with the prior art. Thereason for this is that by forming the gate 8 substantially along theflanks of pyramid 4, a greater degree of control is exercisable over themovement of charge carriers within those regions of the pyramid 4.

In the above examples, the device has been described by reference to apyramid. it will be understood that this is merely illustrative of atapered protrusion, and other structures may equally well be employed,for example cones, needles or the like.

In the above examples, all coatings have been formed completely aroundthe periphery of the pyramid. This is not essential to the presentinvention. A device in accordance with the present invention functionequally well if such coatings are substantially around the periphery ofthe pyramid. It will be apparent that this still permits the appropriatephysical effects to be achieved. Similarly, the degree to which thecoatings enclose the pyramid is arbitrary and to be dictated solely bythe performance desired in the final device. Thus, for example, themetal mayer may extend up to the tip or only half-way between the tipand the base of the pyramid.

Whilst in the above examples, oxide and metal have been illustrative ofan electrical insulator and conductor respectively; it will beappreciated that any suitable material exhibiting the requisite physicalproperties will suffice.

Furthermore, whilst photoresist has been described as illustrative of aplastics material, any material exhibiting suitable plastics properties,i.e. under the baking action, the material is drawn towards the base ofthe pyramid by surface tension 80 as to at least partially expose itstip, will suffice.

Whilst the above examples employ microengineering fabricationtechniques, it must be appreciated that the tip of the pyramid will havea diameter in the range 10⁻⁹ m in order to provide an efficient fieldemission.

It will be appreciated to those skilled in the art that modifications tothe above description are possible whilst still remaining within thescope of the invention, for example, it may be advantageous to have acoating of an oxy-nitride between the oxide and metal layers.

I claim:
 1. A self-aligned gate field emitter device comprising: a substrate (2) carrying a tapered protrusion (4); the tapered protrusion carrying on electrically insulative layer (6) at least partially covering the protrusion, the electrically insulative material extending along the flanks of the tapered protrusion from the base adjacent the substrate towards the tip of the protrusion remote from the substrate;electrically conductive material (8) formed on the electrically insulative layer and extending further towards the tip of the protrusion than the insulative layer and spaced from the protrusion, the tapered protrusion forming the emitter of the device and the electrically conductive material forming the gate of-the device, which gate, in operation of the device, provides control for the level of field emission from the emitter, characterized in that electrically conductive material is partially covered by thermoplastic material (10) substantially around the base of the protrusion for supporting the electrically conductive material.
 2. A device according to claim 1 wherein the thermoplastic material is photoresist.
 3. A device according to claim 1 wherein the electrically insulative material is formed by oxidation of the tapered protrusion.
 4. A device according to claim 3 wherein the tapered protrusion is formed from the substrate material.
 5. A device according to claim 1 wherein the electrically insulative material is an oxide coating formed on the protrusion.
 6. A device according to claim 1 wherein the electrically conductive material is a metal.
 7. A device according to claim 1 wherein the protrusion is a semiconductor.
 8. A device according to claim 7 wherein the semiconductor is silicon.
 9. A device according to claim 7 wherein the semiconductor is doped to be at least partially n-type.
 10. A device according to claim 9 wherein the base and tip regions of the protrusion are n-type and the region therebetween is p-type.
 11. A device according to claim 1 wherein the electrically insulative material further forms a cap on the tip of the protrusion.
 12. A device according to claim 1 wherein the electrically insulative material is overlaid by a layer of oxy-nitride material.
 13. A method of producing a self-aligned gate field emitter device comprising:providing a substrate of material from which the field emitter is to be produced and forming a tapered protrusion thereon; forming, on the surface of the protrusion, electrically insulative material; coating the electrically insulative material with electrically conductive material; at least partially coating the electrically conductive material with thermoplastic material; planarizing the device such that the thermoplastic material remains around the base of the protrusion substantially remote from the tip thereof to at least partially expose the electrically conductive material; selectively removing at least part of the electrically conductive material and the electrically insulative material thereby to define a portion of the device substantially surrounding and enclosing the protrusion characterized in that the planarizing comprises heating the thermoplastic material so that it flows and settles around the base of the protrusion.
 14. A method of producing a self-aligned gate emitter device comprising:providing a substrate of material from which the field emitter is to be produced and forming a tapered protrusion thereon; forming, on the tip of the protrusion a cap of the electrically insulative material and further forming on the surface of the protrusion, electrically insulative material; rotating the device about an axis through the tip of the protrusion and substantially perpendicular to the base thereof; coating the electrically insulative material, off-axis, with electrically conductive material; selectively removing at least part of the electrically conductive material and the electrically insulative material, including the cap, thereby to define a portion of the device substantially surrounding and enclosing the protrusion.
 15. A method according to claims 13 wherein formation of electrically insulative material on the protrusion is achieved by oxidation of the surface of the protrusion.
 16. A method according to claim 13 wherein formation of electrically insulative material on the protrusion is achieved by coating the protrusion with an oxide layer.
 17. A method according to claim 13 wherein the selective removing comprises etching of both the electrically conductive material and the electrically insulative material.
 18. A method according to claim 17 wherein more electrically insulative material is etched than electrically conductive material.
 19. A method according to claims 13 wherein the tapered protrusion is formed from the substrate material.
 20. A method according to claim 13 wherein the electrically conductive material is a metal.
 21. A method according to claim 13 wherein the protrusion is formed from a semiconductor.
 22. A device according to claim 21 wherein the semiconductor is silicon.
 23. A method according to claim 21 wherein the semiconductor is doped to be at least partially n-type.
 24. A method according to claim 21 wherein the base and tip regions of the protrusion are n-type and the region therebetween is p-type. 